1. Field of the Invention
The invention is based on a method for controlling a three-phase inverter in accordance with the preamble of claim 1.
2. Discussion of Background
The preamble of the invention relates to a prior art which is known from CH-A-420.365. In this document, several inverter groups are connected in parallel on the alternating-current side via one or more reactor coils for reducing the harmonic content in each phase in a three-phase inverter circuit for feeding a converter-fed motor, in which sinusoidal alternating voltages corresponding to the required frequency and having a high harmonic content are generated from direct-voltage components of different polarity. One motor winding each is connected to the center of one reactor coil. For controlling the inverter, a sinusoidal voltage of the required motor frequency is superimposed on a triangular voltage of higher frequency than the maximum motor frequency, the triangular voltage having different phase angles of 0.degree. and 180.degree., possibly also of 90.degree. and 270.degree., for the inverter groups connected in parallel on the alternating-current side. The inverter groups of each phase can be connected in series, reactor coils having one winding each for two inverter groups being provided with common iron core. Inverters of different phases can be coupled to one another via reactor coils. Two inverter groups each can be connected to one primary winding each of transformers, the secondary windings of which are connected in series and are connected to an inverter busbar.
With respect to the relevant prior art, reference is furthermore made to CH-A-489,945 in which a similar inverter control method for feeding a variable-speed asynchronous machine is specified.
With each switch-over of the inverter, turn-on and turn-off losses are produced in the converter valves and their damping elements, which must be removed as heat from the semiconductor valves by means of cooling devices. The efficiency of the inverter decreases with increasing inverter switching frequency and the thermal loading of the semiconductor element rises. For these reasons, it is desirable to be able to manage with as low a relative switching frequency as possible. A three-point circuit is known from DE-Al 29 37 995.
From the German journal etz Archiv. Vol. 10 (1988), No. 7, pages 215-220, it is known that the thyristors of a 3-point inverter are 2 converter sections in concept which are in each case operated as 2-point inverters.
From: Control in Power Electronics and Drives, IFAC SYMPOSIUM, Dusseldorf, Oct. 7-9, 1974, Preprints Volume 1, pages 457-472, it is known that in the 2-pulse control method, the unilaterally sinusoidal pulse width modulation has the most advantageous ratio between harmonics in the current and pulse frequency of the inverter in the lower speed range.